HIV-dementia is the most common form of dementia in persons under 40 years of age. Therapeutic interventions designed to improve cognitive function or to prevent further cognitive decline in patients with HIV-dementia have focused on drugs that interact with receptors such as excitatory amino acid receptors and voltage operated calcium channels. Unfortunately, clinical trials with these agents have had limited success. In this proposal we look at a common pathway that is deregulated by HIV infection. We present the first evidence that begins to identify a mechanism whereby increased levels of ceramide and cholesterol in the brains and CSF of HIV infected patients with dementia can lead to neuronal dysfunction and death. We further identify and test potential neuroprotective therapeutics that stabilize sphingolipid metabolism. The long-term goals of this research are to discover and test therapeutic agents that protect neuronal function by stabilizing sphingolipid biochemistry. Sphingomyelin, ceramide, cholesterol and ganglosides are the primary constituents of specialized membrane domains called 'lipid rafts". These specialized regions of cellular membranes are thought be important to coordinate cellular signaling by localizing functional groups of proteins in the plasma membrane and by coupling transmembrane receptors with signal transduction machinery. Lipid rafts can be modified by ceramide to form larger domains, which serve to cluster receptor molecules. The generation of a high receptor density might be required for initiation of receptor-specific signaling and has been implicated as pivotal step in the formation of "death domains". We have identified a ceramide-dependent mechanism that promotes the clustering of N-methyl-D- aspartate (NMDA) receptors into lipid rafts in response to the neurotoxic HIV-1 proteins gp120 and Tat. NMDA-evoked calcium bursts in these microdomains are sufficiently elevated to activate calcium dependent death effectors. Using biochemical, biophysical, molecular, electrophysiological and imaging techniques we propose to determine the mechanisms that direct the neurotoxic effects of the HIV-1 proteins gp120 and Tat by disrupting sphingolipid metabolism. These findings may lead to the rational design of pharmaceutical agents that are neuroprotective by mechanisms that stabilize sphingolipid metabolism and prevent the abnormal clustering of dysfunctional NMDA receptors.